1. Field of the Invention
The present invention relates to a noise suppressor used in an apparatus having a cooling fan and a cooling duct, to an electronic apparatus provided with a noise suppressor, and to a method of effectively controlling the noise suppression characteristics of these devices, and more particularly, relates to a noise suppressor and method for effectively suppressing the noise of a cooling fan used in a projection display device such as a liquid crystal projector.
2. Description of the Related Art
Projection display devices that enlarge and project upon a screen images generated on image display elements are becoming widely used not only in the workplace but also in typical homes. Among such projection display devices, a liquid crystal projector that employs a liquid crystal panel as the image display element displays an image on a screen as described herein after.
White light that is emitted from a light source is reflected by a reflector and then undergoes polarization conversion, and further, separation into each of the colors R, G, and B. Each of the separated colors is irradiated onto a corresponding liquid crystal panel in which each of the colors is subjected to light modulation according to a video signal by means of the corresponding liquid crystal panel. The light of each of the colors that has undergone light modulation is then synthesized in a color-synthesizing prism and projected onto a screen by way of optical system for projection.
When a TN (Twisted Nematic) liquid crystal panel is used as the liquid crystal panel, the TN liquid crystal panel can handle only specific linearly polarized light components, and the direction of polarization of the light of each color is therefore aligned in a prescribed direction of polarization (P-polarization) at a polarizer film (for example, a P-polarizer film) on the side of light incidence. The P-polarization component of the light that has undergone light modulation by the liquid crystal panel is then cut by a polarizer film (S-polarizer film) on the side of light emission that differs from the polarizer film on the side of light incidence, whereby only the S-polarization component is extracted.
In a light modulation module of the above-described configuration, an incident-side polarizer film and an emission-side polarizer film that are disposed before and after the liquid crystal panel and that together with the liquid crystal panel make up the liquid crystal unit pass, only polarize light of one axial direction and block other polarized light. The incident-side sheet polarizer and emission-side sheet polarizer are, therefore, prone to heating that results from light absorption.
In addition, a black matrix is provided at each pixel boundary of a liquid crystal panel, and the blockage of transmitted light in this black matrix generates heat to which the liquid crystal panel adds heat generated during its operation.
Organic materials are frequently employed for the liquid crystal panel and sheet polarizes. When irradiated by light of ultraviolet light (UV) and exposed to high temperatures over long periods of operation, the performance of these components suffers considerable deterioration such as damage to the panel alignment layer and loss of the polarization selection characteristic. These light modulation modules therefore call for heat countermeasures such as forced-air cooling.
FIG. 1(a) shows the external appearance of typical liquid crystal projector 1a, and FIG. 1(b) shows the internal configuration of liquid crystal projector 1a. FIG. 2 gives a schematic representation of an example of the internal configuration of liquid crystal projector 1a. 
As shown in FIG. 2, first sirocco fan 3 and first cooling duct 4 for effecting forced-air cooling of light modulation module 2a and second sirocco fan 6 and second cooling duct 7 for effecting forced-air cooling of lamp bulb 5 are installed in the enclosure of liquid crystal projector 1a. In addition, an exhaust fan (not shown) is sometimes further provided for exhausting air that has been heated to a high temperature within the enclosure to the outside. In addition to these components, a fan for cooling power supply unit 9 is sometimes provided according to necessity.
Explanation here regards the configuration for cooling the light modulation module of a typical liquid crystal projector using FIG. 3 and FIG. 4. In FIG. 3(a), the optics engine components of the liquid crystal projector in FIG. 1(b) are extracted, and FIG. 3(b) shows an exploded view of the cooling system of this light modulation module 2a. 
The cooling module of light modulation module 2a in FIG. 3(b) is made up from first sirocco fan 3 and first cooling duct 4, and as shown in the sectional view of FIG. 4, forced-air cooling is realized by passing cooling air 16 from first sirocco fan 3 by way of duct exhaust ports 12 provided in first cooling duct 4 from the lower end of light modulation module 2a and through each of the R/B/G liquid crystal units (incident-side sheet polarizer 13/liquid crystal panel 14/emission-side sheet polarizer 15).
Recent years have seen growing demand for liquid crystal projectors having sizes that are more compact and higher brightness. Increase of lamp output and downsizing of the display device have accelerated to meet these demands, and as a result, the luminous flux density of light that is irradiated into image display elements (the liquid crystal unit) is increasing and the heat load upon the device is steadily rising.
For example, in a liquid crystal projector of the 2000 lm class, the total heat generation is in the range of 15 W and the heat flux of the emission-side sheet polarizer is 0.6 W/cm2. In the 5000 lm class, however, the total heat generation of the liquid crystal unit rises to 35 W or more, and the heat flux of the emission-side sheet polarizer reaches 1.4 W/cm2 or more.
When the forced-air cooling method is adopted for cooling, the increased heat load is dealt with by increasing the amount of air from the fan and raising the wind velocity around the source of heat generation to raise the efficiency of heat transfer and improve cooling performance.
However, attempting to increase air flow by raising the fan revolutions leads to increased operation noise. To address this problem, quieter operation is attempted by using a larger fan at lower revolutions or by employing ventilation ducts having a high sound-deadening effect.
FIGS. 5(a) and 5(b) show perspective and sectional views of the construction of a prior-art example of a lined duct dissipative muffler. This lined duct dissipative muffler 17 includes ventilation duct 19a provided with fan 18 at one longitudinal end, and porous sound-absorbing material 20a such as glass wool that lines the inner surface of the duct. Sound that is propagated through ventilation duct 19a enters the medium (porous sound-absorbing material 20a) and is damped by the viscous damping of the aerial vibration within the fiber material and the conversion from sound energy to heat energy caused by movement of the fibers.
FIG. 6 shows an example of a lined elbow dissipative muffler of the prior art. This lined elbow dissipative muffler 22 includes bent duct 23 provided with fan 18 at one end and porous sound-absorbing material 20b that lines the inside of bent duct 23. This type of dissipative muffler can obtain both a sound reduction effect due to the phase interference between incident waves and reflected waves at the bent portion of bent duct 23 and a sound-reduction effect due to the porous sound-absorbing material for sound that is dispersed at the bent portion.
JP-A-2001-68882 discloses a projector device that is provided with the above-described lined elbow dissipative muffler. More specifically, a projector is disclosed that is provided with bent intake and exhaust ducts in which porous sound-absorbing material lines the inner surfaces.
FIG. 7 shows an example of an active noise control (ANC) muffler of the prior art. This active muffler 24 includes detection microphone 25, controller 26, amplifier 27, speaker 28, and error microphone 29.
Detection microphone 25 supplies a signal according to noise in ventilation duct 19b, and controller 26 analyzes the signal supplied from detection microphone 25 and generates a signal of the opposite phase of this signal.
Amplifier 27 amplifies the signal generated by controller 26, and speaker 28 generates sound according to the signal that has been amplified by amplifier 27. Error microphone 29 verifies whether the noise (sound waves) in the duct and the sound (sound waves) generated by the speaker cancel each other out and feeds back the result to controller 26.
As will be clear from the foregoing explanation, this type of noise suppressor uses sound wave interference produced by a secondary sound source to damp noise. JP-A-06-282278 discloses an example of the same type of noise suppressor, i.e., an active noise suppressor that eliminates the noise of a standing wave generated in a duct. In addition, several examples have been reported that use resonator mufflers that employ Helmholtz resonators to suppress sound.
JP-A-2001-92468 discloses a sound insulation wall and a method of designing a sound insulation wall.
In this sound insulation wall, a sound insulation wall main body is composed of a pair of planar main walls that confront each other over a spacing and sub-walls that join the outer peripheries of the two main walls and form an interior space between the two main walls, and tubes that pass through the two main walls and that form air passages are provided at substantially uniform spacing between the two main walls.
Communicating holes are provided in these tubes that allow the air passages to communicate with the interior space. When designing this sound insulation wall, the volume of the interior space and the number of air passages are determined in accordance with the relation equation between the central frequency of noise and the speed of sound so as to raise the sound attenuation effect in the Helmholtz resonator formed by the communicating holes and the back space of the air passages.
The following explanation is regarding the sound absorbing action and resonation principles of a Helmholtz resonator. FIG. 8 is a schematic view of the basic configuration of a Helmholtz resonator.
Helmholtz resonator 30 has a construction in which cavity portion 31 having a large volume VO is provided with small neck portion 32. When sound (noise) of a frequency that matches the resonance frequency of the air spring vibration of this construction passes through neck portion 32 and reaches cavity portion 31, a resonance phenomenon occurs by which the air of neck portion 32 vibrates violently, converting a portion of the sound energy to heat energy by viscous loss and producing a sound-absorbing effect.
As a sound-absorbing construction that uses the above-described Helmholtz resonance principle, a perforated plate construction with a back air layer such as shown in FIG. 9 is well known.
In perforated plate construction 33 shown in FIG. 9, perforated plate 35 having a large number of through-holes 34 is secured to spacing walls 90 separated by a distance L1 from wall surface 83. Cavity volume V1 is formed between this perforated plate 35 and wall surface 83 and, together with through-holes 34 in perforated plate 35, forms a Helmholtz resonator. The chief factors that determine the sound-absorbing characteristics of the perforated plate construction in this case include the specifications of the perforated plate (plate thickness, diameter of through-holes, and hole pitch) and conditions of use (thickness of the back air layer and foundation conditions), and the material of the plate has no effect on the sound-absorbing characteristic.
Of each of the previously described factors, the specifications of perforated plate 35 and the thickness L1 of the back air layer relate to the resonance frequency at which the sound-absorption coefficient reaches a maximum, and the foundation material relates to the magnitude of the sound-absorption coefficient. The foundation material described here is the porous material (glass wool or felt) that is applied to the back-space layer side of perforated plate 35. If the thickness of the back space layer is no greater than 500 mm, the resonance frequency fr that determines the main sound-absorption region is calculated by the following equation:
                              f          r                =                              c                          2              ⁢                                                          ⁢              π                                ⁢                                    P                                                (                                                            t                      1                                        +                                          0.8                      ⁢                                                                                          ⁢                                              d                        1                                                                              )                                ×                                  L                  1                                                                                        (        1        )            where fr is the resonance frequency (Hz), c is the speed of sound in air (m/s), P is the open area ratio, t1 is the plate thickness (m), d1 is the diameter of through-holes (m), and L1 is the thickness of the back air layer (m).
As another example in which a Helmholtz resonator is applied as a muffler, a construction is also known in which, as shown in FIG. 10, a portion of a duct is formed as a duplex tube, inner passage tube 37 provided with a plurality of through-holes 39a and outer closed pipe 38 together forming perforated-pipe resonator muffler 36.
JP-A-2001-222065 discloses an example that aims for a sound-absorbing effect by arranging the above-described perforated pipe resonator muffler in a ventilation duct. FIG. 11 shows the schematic configuration of the resonator muffler in a duct for cooling a liquid crystal panel of the projector disclosed in JP-A-2001-222065.
As shown in FIG. 11, JP-A-2001-222065 discloses a first resonator muffler 43 composed of first resonance chamber 41 and through-holes 39b and second resonator muffler 44 composed of second resonation chamber 42 and through-holes 39c, which are constructed in both the air-intake side and air-output side of liquid crystal panel cooling duct 40. The operation noise of cooling fan 45 that occurs inside liquid crystal panel cooling duct 40 is damped by the resonance phenomenon of resonator mufflers 43 and 44 to suppress the sound.
In addition, JP-A-2005-30308 discloses a method for variable control of the sound-absorption frequency of a Helmholtz resonator provided in an intake duct. FIGS. 12(a) and 12(b) show the schematic configuration of the resonator noise suppressor in the intake duct disclosed in JP-A-2005-30308. As shown in FIGS. 12(a) and 12(b), JP-A-2005-30308 discloses a branch pipe 86 which is provided midway through duct 85 which is the intake/exhaust passage, and thus provides a connection to resonance box 87, and mechanically rotating fan-shaped movable plate 88 provided in the neck portion enables continuous variation of the area of the profile of the neck portion and control for obtaining any resonance frequency.